This invention relates to a process for identifying and isolating quartz, quartzite, glass or silicate materials having selected aluminum contents. More particularly, the invention relates to a non-destructive method for determining the aluminum contents of said materials.
Aluminum is an important impurity which is found in quartz, quartzite, glass or silicate materials. In natural quartz, aluminum is one of the major impurities, the others being iron, titanium, sodium, lithium and hydrogen. The impurities in, for example, natural quartz, affect products made from it, both in cultured quartz growing and in the manufacturing of fused quartz, where high-purity products are often desired. It has been possible to segregate many of the impurities other than aluminum by physically eliminating from the lump all foreign materials adhering to the lump surfaces or included within the lumps. Most iron and titanium are effectively removed by this method. The main surface-absorbed impurities, sodium, lithium and metallic iron (but not their dissolved fractions) may then be removed by an acid wash. In the fused quartz industry, acids such as hydrochloric and hydrofluoric acid are commonly used for such an acid wash.
Aluminum, however, has been particularly difficult to revome as an impuirty in quartz because it is usually dissolved in the quartz and pure lumps cannot (until now) be separated from impure lumps by any known, non-destructive, practical technique.
The presence of aluminum in raw natural quartz has been a problem in cultured quartz growing processes because the aluminum dissolves in the hydrothermal fluid and affects several properties of the crystals grown from that fluid. The presence of aluminum in the fluid affects the following crystal properties as indicated: Q is decreased (see IEEE Trans. on Sonics and Ultrasonics, Vol. Su-19, No. 1, January, 1972, pages 41-44); the relative sizes of chamfer or S faces are increased; X/Z growth ratio is increased; and the tendency toward creating crevicing flaws is increased. While lithium doping of the hydrothermal solution can be used to control partially the first two of these undesirable properties, sorting of the raw material to control the aluminum content within acceptable appropriate tolerance limits remains the most desirable control method for these problems.
In the fused quartz industry, the control of the variable aluminum content is equally, or more, important. For example, where the fused quartz crystals are to be applied to optical uses, the fused quartz must be homogeneous. It has been observed that if lumps or particles of quartz having different aluminum contents are fused together, the resulting fused quartz will not have a uniform optical index, and the material cannot be used without a long and costly final anneal.
Another type of impurity which can cause problems in fused quartz applications is the alkali metals. It has been found that alkali-bearing quartz is more prone to devitrification. Therefore, in fused quartz applications where devitrification is a problem, uncontrolled alkali-bearing lumps of quartz should not be utilized.
It has long been known that natural quartz crystals turn smoky under the influence of X-rays or gamma rays, some more and some less. Kats (Thesis, Delft (1961), see also "Hydrogen in Alpha-Quartz," Philips Research Reports, 17:201-279 (June, 1962) appears to be the first to explicitly describe the yellow shade that some lumps develop, and to note that these contained more OH than the others. Bambauer (Schweiz. Min. Petr. Mitt., 41:335 (1961) studied the relationships of the saturated colors to impurities and stated that the aluminum ions were compensated by the sum of monovalent impurities as in Al=Na + Li + H.
Lehman studied further the yellow color center called "honey" ("Yellow Color Centers in Natural and Synthetic Quartz," Phys, kondens. Materie, 13:297-306 (1971).